Common hydrogels absorb no more than 100% (1 g water/1 g hydrogel). Superabsorbent materials are hydrophilic polymer networks which can absorb water from 1000-100,000% (10 to 1000 g/g) their dry weight, with typical values on the order of 1000 to 30,000% (10 to 300 g/g). Hydrogels are water insoluble in aqueous environment because of chemical or physical crosslinking between polymer chains. In physically crosslinked hydrogels, polymer network is held together by molecular entanglements or secondary forces including hydrogen bonding or hydrophobic forces. Chemically crosslinked hydrogels are covalently bonded between different polymer chains within the network.
Hydrogels have received a great deal of attention and significant progress has been made in development of these materials for many biological and biomedical applications as well as use as superabsorbents.
Superabsorbent materials are water insoluble and hydrophilic polymer networks which take up water from 10-1000 times of their dry weight. Hydrogels are receiving increasing attention because of their ability to retain great quantity of water and good biocompatibility. Hydrogels may be chemically stable or degrade eventually and dissolve. When a dry hydrogel begins to absorb water, the first water molecules entering the matrix will hydrate the most polar, hydrophilic groups, leading to “primary bound water”. As the polar groups are hydrated, the network swells and exposes hydrophobic groups, which also interact with water molecules, leading to “secondary bound water”. Primary and secondary bound water are often combined and simply called the “total bound water”. After the polar and hydrophobic sites have interacted with and bound water molecules, the network will imbibe additional water, due to the osmotic driving force of the network chains towards infinite dilution. This additional swelling is opposed by the covalent or physical crosslinks, leading to an elastic network retraction force. Thus, the hydrogel will reach an equilibrium swelling level. The additional swelling water that is imbibed after the ionic, polar and hydrophobic groups become saturated with bound water is called “free water” or “bulk water”, and is assumed to fill the space between the network chains, and/or the center of larger pores, macropores or voids. As the network swells, if the network chains or crosslinks are degradable, the gel will begin to disintegrate and dissolve, at a rate depending on its composition. However, it has been reported that swelling in aqueous solutions containing salts typically found in physiological fluids cause the swelling to be reduced by as much as 30%. Superabsorbent materials are typically capable of absorbing about 30 g/g in an aqueous solution containing 0.9 weight percent sodium chloride solution in water.
Thus there remains a need for additional hydrogel compositions that absorb a greater percentage of physiological fluids. Additionally there is a need for biodegradable materials for use as superabsorbents, since the current acrylic-based superabsorbents are not biodegradable.